A word from Britt, the 'Spongebob Squarepants' costume creator:

My son, Timlend, adores Spongebob Squarepants. He already had a white shirt, brown pants, and black shoes in his wardrobe to wear underneath the outer costume.

The square body: I used an old cardboard box appropriate for his height and width of his shoulders. I cut out two arms holes and a large opening for the mouth so my son could see out. I used yellow duct tape to cover the entire box.
Using a multi-colored pack of sticky foam sheets to create depth, I fashioned his big blue eyes, black lashes, yellow nose and sponge “holes”, pink tongue, red tie, white shirt, and brown pants.
The hat: I cut an anchor out of blue foam and stuck it on an old white cap to serve as his work hat at the Krusty Krab.

The cost: Yellow duct tape is about $3-$5. A pack of sticky foam sheets is about $5-$7.

My son was so happy he got to dress up as his favorite cartoon character and I scored major mom points for creating it.